In certain machines, such as automatic drilling machines, routers or combination drillers and routers used in the printed circuit board industry, it is common practice to provide a spindle motor that drives the cutting tool at high rotational speeds, such as 60,000 rpm. In these motors, particularly because of their rotational velocities, it is critical to obtain precise alignment of the two bearings which support the armature assembly. With conventional manufacturing techniques, there remains a degree of error in bearing alignment which necessitates a protracted period of run-in before the motor is suitable for use. Bearing life also may be cut short by lack of extreme accuracy in alignment.
In order to retain the field winding within the housing of the motor and to provide support for the bearings, the motor housing must be formed in at least two pieces. In the prior art, it has been common to attach the two housing sections together by means of bolts or screws. Each housing section is produced separately from the other, including the formation of bearing support surfaces of the housing. Inherently, this leads to some inaccuracy in the alignment of the bearing support surfaces in the housing. Moreover, when the housing sections are secured together by fasteners, such as bolts or screws, the tensioning of the fasteners introduces unequal stresses into the housing, causing the housing to become distorted, thereby contributing to the misalignment of the bearing support surfaces.
In the prior art, it has been common to provide for the flow of coolant through the motor to remove heat from the bearing areas and prevent their destruction from heat build-up. This has been accomplished by a spiral passage through the motor for the flow of coolant. This type of passage creates a restriction which impedes the flow and therefore is undesirable.